Monopolar probes are intended for the coarse stimulation of tissue to determine whether there is neural tissue lying on the surface of, or within, a tumour mass – typical usage would be to run a monopolar probe over the surface of a CPA tumor once the dura has been opened. Monopolar probes can also be used to stimulate more accurately if the stimulating current is closely controlled, but bearing in mind that the current will always take the easiest path back to the return electrode, which may lead to unexpected results. Hence, monopolar probes will stimulate through the bony canal in mastoid surgery, the pedicle wall in spinal surgery, and can be used to map nerves if the current is reduced to a value just above threshold level